A high-throughput, retrovirus-mediated mutagenesis method based on gene trapping in embryonic stem cells was used to identify a novel mouse gene. The human ortholog encodes a transmembrane protein containing five extracellular immunoglobulin-like domains that is structurally related to human NEPHRIN, a protein associated with congenital nephrotic syndrome. Northern analysis revealed wide expression in humans and mice, with highest expression in kidney. Based on similarity to NEPHRIN and abundant expression in kidney, this protein was designated NEPH1 and embryonic stem cells containing the retroviral insertion in the Neph1 locus were used to generate mutant mice. Analysis of kidney RNA fromNeph1^−/− mice showed that the retroviral insertion disrupted expression of Neph1 transcripts.Neph1^−/− pups were represented at the expected normal Mendelian ratios at 1 to 3 days of age but at only 10% of the expected frequency at 10 to 12 days after birth, suggesting an early postnatal lethality. The Neph1^−/−animals that survived beyond the first week of life were sickly and small but without edema, and all died between 3 and 8 weeks of age. Proteinuria ranging from 300 to 2,000 mg/dl was present in allNeph1^−/− mice. Electron microscopy demonstrated NEPH1 expression in glomerular podocytes and revealed effacement of podocyte foot processes in Neph1^−/− mice. These findings suggest that NEPH1, like NEPHRIN, may play an important role in maintaining the structure of the filtration barrier that prevents proteins from freely entering the glomerular urinary space.